1. Field of the Invention
The present invention relates to a Y/C separation circuit for separating a composite video signal into a luminance signal and a chrominance signal.
2. Description of the Related Art
There is known a Y/C separation circuit using a comb filter such as, for example, disclosed in Japanese Patent Application No. 2-137081.
FIG. 5 shows a Y/C separation circuit using a comb filter of a conventional technique. A 1 H (a period of one horizontal synchronization signal, or, briefly, one horizontal scanning period) delay line 4F outputs a composite video signal b obtained by delaying an input composite video signal a thereto by one horizontal scanning period, while a 1 H delay line 4S outputs a composite video signal c obtained by delaying the composite video signal b output from the 1 H delay line 4F by one horizontal scanning period. The input composite video signal a, the composite video signal b output from the 1 H delay line 4F, and the composite video signal c output from the 1 H delay line 4S correspond to the composite video signals for first, second, and third horizontal lines.
A bandpass filter (BPF) 6 extracts a signal d as a high-frequency component of the input composite video signal a. A bandpass filter 8 extracts a signal e as a high-frequency component of the composite video signal b output from the 1 H delay line 4F. A bandpass filter 10 extracts a signal f as a high-frequency component of the composite video signal c output from the 1 H delay line 4S.
A subtractor 122 subtracts the output signal d of the BPF 6 from the output signal e of the BPF 8, and a 1/2 (multiply by a half) circuit 124 outputs a signal g obtained by halving the output of the subtractor 122. A subtractor 142 subtracts the output signal f of the BPF 10 from the output signal e of the BPF 8, and a 1/2 circuit 144 outputs a signal h obtained by halving the output of the subtractor 142. An adder 162 adds up the signal g and the signal h, and a 1/2 circuit 164 outputs a signal obtained by halving the output signal of the adder 162, i.e., a chrominance signal i. A subtractor 170 subtracts the chrominance signal i from the composite video signal b output from the 1 H delay line 4F and outputs the difference as a luminance signal j.
In the circuit of the conventional structure shown in FIG. 5, while the level of a random noise in the luminance signal j is reduced to 1/2, if a granular random noise is contained in the composite video signal a, there are produced noises in the outputs e, d, and f of the BPFs 8, 6, and 10 as shown in FIG. 6, FIG. 7, and FIG. 8, and as a result the random noise is spread over three horizontal lines vertically located in succession. Thus, there has been a problem that, while the noise level is lowered, the noise frequency is trebled.
When an NTSC signal is converted into a PAL signal in a broadcasting station, sometimes the carrier signal of the NTSC signal leaks out and becomes a low-level vertical line. Since this carrier signal is output from the j terminal the same as the luminance signal, it has been unable to be eliminated by the conventional circuit shown in FIG. 5.
When a trap filter is used instead of the above mentioned comb filter, an increase in noise can be prevented, but there arises a problem that the high-frequency component of the luminance signal becomes deteriorated.